Single-crystal hexaborides and method of preparation

ABSTRACT

An aluminum flux method of preparing binary, ternary, and quaternary single crystal metal hexaborides. The single crystals are prepared by mixing a metal oxide, carbonate, or nitrate compound with boron powder in an amount of aluminum which will solubilize the boron at a reaction temperature of from about 1200° C. to about 1600° C. The mixture is held at the reaction temperature for a sufficient time to form the desired single crystal hexaboride.

This invention relates to a method of forming binary, ternary, and quaternary metal hexaborides. More specifically, the invention relates to an aluminum flux method of preparing binary, ternary, quaternary single crystal metal hexaborides.

BACKGROUND OF THE INVENTION

Methods for the preparation of lanthanum hexaboride, europium hexaboride and europium yttrium hexaboride crystals are known. The crystals have been used as cathodes for thermionic emission and field emission devices. The crystals have been produced basically by two methods.

A hot pressing technique described by Kawabe et al in U.S. pat. No. 3,932,314 to form (Y_(1-x) Eu_(x))B₆ comprises, inter alia, mixing the oxides Y₂ O₃ and Eu₂ O₃ with boron powder and polyvinyl alcohol as a molding agent and thereafter applying a pressure of about 1 ton/cm² to compact the powders with subsequent heating in a boron nitride crucible at a temperature of about 1525° C. for 1 hour in a vacuum of 10⁻² to about 10⁻³ Torr.

Tanaka et al, J. Crystal Growth, 40 (1977), pp. 125-128, describe the formation of EuB₆ by a borothermal reduction according to the following equation:

    Eu.sub.2 O.sub.3 +15B→2EuB.sub.6 +3BO

The borothermal formation of EuB₆ is similar to the work of Kawabe et al in that it involves the reaction of Eu₂ O₃ with boron powder, high pressure compressing and subsequent heating. The hot pressing technique tends to form polycrystalline materials which may incorporate binder impurities into the crystal which can degrade the performance of the hexaboride material.

U.S. Pat. No. 4,054,946 describes the formation of LaB₆ single crystals by an aluminum flux technique. High purity lanthanum metal, boron powder and high purity aluminum are mixed together and heated to from about 1200° to about 1600° C. from a time period which may vary from several minutes to several days to produce LaB₆ which precipitates from the aluminum flux as a single crystal compound upon cooling of the mixture. Thereafter the aluminum flux is dissolved with a strong acid such as HCl. Futamoto et al, Japan J. Appl. Phys., Vol. 14, No. 9, 1975, and Aita et al, Japan J. Appl. Phys., Vol. 13, pp. 1263-1266, No. 2, (1974), teach similar aluminum flux techniques.

The aluminum flux technique permits the production of single crystal hexaborides. However, the reactants involved, namely lanthanum or other lanthanide metals, tend to be spontaneously combustible when exposed to oxygen and therefore, the reactions must be carried out under an inert atmosphere. The extra care and precautions which must be taken when growing single crystal hexaborides by the above technique greatly add to the cost of production.

Thus, it would be highly desirable to have a technique which can produce, binary, ternary and quaternary single crystal metal hexaborides without the impurities associated with hot pressing or the expensive precautions necessary to prevent explosions during the crystal growing process.

SUMMARY OF THE INVENTION

We have invented a method for growing binary, ternary, and quaternary single crystal hexaborides which comprises mixing with aluminum quantities of one or more metal compounds which can be the oxide, nitride or carbonate, with boron. The boron is present in an amount which is soluble in the aluminum at the crystal growing temperature. The mixture is heated to a temperature of from about 1200° C. to about 1600° C. for a sufficient time to form the single crystal hexaboride. Thereafter, the aluminum is dissolved away from the single crystal hexaboride with a strong acid such as HCl or a strong base such as NaOH. The crystals are useful as cathodes for thermionic emission devices.

DETAILED DESCRIPTION OF THE INVENTION

The binary, ternary and quaternary single-crystal hexaborides which can be grown according to my method, can be illustrated by the formula:

    L.sub.x M.sub.z R.sub.1-x-z B.sub.6

where L is selected from the group consisting of cesium (Cs), barium (Ba) and the lanthanides. Preferred lanthanides are lanthanum (La), cerium (Ce) and europium (Eu). M and R are selected from the group consisting of Cs, Ba, La, yttrium (Y) and the lanthanides. Preferably M and R are selected from the group consisting of La, Ce, Eu, Cs, Ba and Y. X is greater than 0 but less than or equal to 1, z is greater than or equal to 0 but less than 1, and 1-x-z is less than 1.

When x is equal to 1 and z is equal to 0, the formula reduces to the binary single crystal hexaborides LB₆. When 1-x-z is equal to 0, and z is greater than0, the formula reduces to the ternary single crystal hexaborides L_(x) M_(z) B₆, Examples of ternary single crystal hexaborides are, inter alia, La_(x) Eu_(z) B₆, La_(x) Y_(z) B₆, La_(x) Cs_(z) B₆, Eu_(x) Y_(z) B₆ and Eu_(x) Ba_(z) B₆. When both x and z are greater than zero, the formula illustrates quaternary single crystal hexaborides subject to the limitations previously given for x and z. Examples of single crystal hexaborides are, inter alia, La_(x) Eu_(z) Y_(1-x-z) B₆ and La_(x) Eu_(z) Ba_(1-x-z) B₆.

The method for growing a binary, ternary, or quaternary single-crystal hexaboride is exemplified by the following reaction for lanthanum hexaboride (LaB₆): ##EQU1## The aluminum, which is in a molten state at the reaction temperature, is used as the growth medium, i.e., flux, for the metal compound and boron mixture.

The single crystal hexaborides are formed by mixing the oxide, carbonate or nitrate metal compound or compounds with boron powder. That amount of boron can be present which is soluble in the aluminum flux at the reaction temperature. The solubility of the boron in aluminum increases with an increase in the reaction temperature and therefore, larger and greater amounts of single-crystal hexaborides can be grown in the same amount of aluminum flux at higher reaction temperatures. The solubility of boron in aluminum is about 1% at about 1200° C., 2% at about 1350° C. and about 3% at 1500° C.

The reactants of the desired single-crystal hexaboride are mixed together and placed in a rectangular alumina boat. The alumina boat containing the reactants is heated to a reaction temperature of from about 1200° C. to about 1600° C. in an inert atmosphere of argon, nitrogen, helium, or like gases. The reaction temperature is maintained for a sufficient time to form the single-crystal hexaboride. Depending upon the size of the crystals desired and the amount of reactants, the reaction time is from about 5 hours to about 5 days. Thereafter, the crystals contained in the aluminum flux are cooled to 600° C. at a linear rate of from about 5° to about 20° C. per hour. Although not critical, the temperature of the furnace during the reaction period is maintained within 1 degree C./hr. Alternatively, the furnace can be cooled exponentially according to the following formula:

    T=T.sub.o e.sup.-ατ

wherein "T" is the temperature, "α" is the cooling rate, T_(o) is the initial temperature, and "τ" is the time of cooling. Longer crystals are obtained when the reaction mixture is exponentially cooled. For example, an initial exponential cooling rate of about 15° C./hr. from about 1500° C. produces longer crystals than linear cooling of the reaction mixture from 1500° C. After cooling, the single-crystal hexaborides are separated from the aluminum matrix with an acid such as HCl.

Ternary as well as quaternary single crystals can be produced according to my method. However, no precise correlations can be made between starting oxide powder ratios and crystal composition since a range of compositions often results. The crystals formed by the method usually have a single composition that is within 15 mole percent of the powder composition. Ternary single crystals produced according to my invention tend to be richer in one metal then the starting mixture according to the descending order: La, Eu, Ba and Y. In other words, equi-molar mixtures of Eu₂ O₃ and BaCO₃ will produce Eu-rich crystals. However, a mixture of 90 weight percent La₂ O₃ powder and 10 weight percent CsNO₃ powder formed a single crystal with a structure similar to LaB₆ and had a concentration of cesium of only 0.1 weight percent as determined by atomic absorption measurements. The following examples illustrate the method according to my invention of preparing binary, ternary, quaternary single-crystal hexaborides. However, it is not my intention to limit my invention solely to the examples described hereafter. I intend to include within the scope of my invention such modifications as would be obvious to the ordinary worker skilled in the art of preparing metal hexaborides.

EXAMPLE 1

LaB₆ single crystals were formed by mixing 22.5167 grams of aluminum having a purity of 99.999% with 0.4503 gram of 99.999% pure boron powder and 1.1302 grams of La₂ O₃ having a purity of 99.999%.

The mixture was placed in a rectangular alumina boat 15 cm² and placed in a furnace. The mixture was heated to about 1400° C. for about 5 days and thereafter cooled at a rate of 8° C. per hour. Upon cooling to room temperature, the solidified mixture was etched in concentrated HCl for a sufficient time to dissolve away the aluminum flux. LaB₆ crystals having a size of about 1.0 mm by 1.0 mm by 3 mm were recovered. The X-ray diffraction pattern of the above compound yielded a lattice parameter of 4.1568, corresponding to LaB₆. The crystal was spot welded to a rhenium wire through which a current was passed to heat the crystal to about 1180° C. A current density of 1.0 amp/cm² was measured.

EXAMPLES 2-4

The following examples of binary single crystals were prepared in accordance with the procedure outlined in Example 1. All weights are in grams unless otherwise indicated.

    ______________________________________                                         BINARY HEXABORIDES                                                             Example        2         3         4                                           ______________________________________                                         Theoretical Crystal                                                                           EuB.sub.6 BaB.sub.6 CeB.sub.6                                   Composition                                                                    Al             23.4379   21.1060   23.5680                                     B              0.4688    0.6332    0.2340                                      Eu.sub.2 O.sub.3                                                                              1.2727    --        --                                          BaCO.sub.3     --        1.9250    --                                          Ce.sub.2 O.sub.3                                                                              --        --        0.5880                                      Crystal Produced                                                                              EuB.sub.6 BaB.sub.6 CeB.sub.6                                   ______________________________________                                    

EXAMPLES 5-16

The following ternary single-crystal hexaborides were prepared in accordance with the procedure outlined in Example 1, with the following exceptions. Example 5 was reacted at about 1400° C. and cooled at a rate of about 8° C./hr. Examples 6, 7, 14 and 16 were reacted at about 1400° C. and cooled at a rate of about 5° C./hr. Examples 8 and 12 were reacted at about 1500° C. and cooled at a rate of about 15° C./hr. Example 9 was reacted at about 1500° C. and cooled exponentially starting with a rate of 15° C./hr. Example 15 was reacted at 1500° C. and cooled at a rate of about 20° C./hr. Examples 10, 11, 12 and 13 were reacted at about 1360° C. and cooled at a rate of about 5° C./hr. The chemical analysis of Examples 11 and 13 indicated the presence of lanthanum, yttrium, and boron but the X-ray and electron probe analysis were unable to give a specific crystal structure.

    __________________________________________________________________________     TERNARY HEXABORIDES                                                            __________________________________________________________________________     Example   5      6      7      8      9                                        __________________________________________________________________________     Theoretical Crystal                                                                      La.sub..5 Eu.sub..5 B.sub.6                                                           La.sub..8 Eu.sub..2 B.sub.6                                                           La.sub..2 Eu.sub..8 B.sub.6                                                           La.sub..5 Ba.sub..5 B.sub.6                                                           La.sub..9 Cs.sub..1 B.sub.6              Composition                                                                    Al        25.4600                                                                               25.7140                                                                               20.8359                                                                               24.9844                                                                               25.5503                                  B         0.5192 0.5143 0.4167 0.4997 0.7665                                   La.sub.2 O.sub.3                                                                         0.6490 1.0286 0.2084 0.6246 1.7323                                   Eu.sub.2 O.sub.3                                                                         0.7009 0.2829 0.8959 --     --                                       BaCO.sub.3                                                                               --     --     --     0.7745 --                                       CsNO.sub.3                                                                               --     --     --     --     0.2300                                   Y.sub.2 O.sub.3                                                                          --     --     --     --     --                                       Crystal Produced                                                                         La.sub..23 Eu.sub..74 B.sub.6                                                         La.sub..91 Eu.sub..09 B.sub.6                                                         Eu.sub..37 Eu.sub..63 B.sub.6                                                         La.sub..52 Ba.sub..48 B.sub.6                                                         La.sub..999 Cs.sub..001 B.sub.6          __________________________________________________________________________     Example   10     11     12     13     14                                       __________________________________________________________________________     Theoretical Crystal                                                                      La.sub..9 Y.sub..1 B.sub.6                                                            La.sub..2 Y.sub..8 B.sub.6                                                            La.sub..5 Y.sub..5 B.sub.6                                                            La.sub..4 Y.sub..6 B.sub.6                                                            Eu.sub..5 Ba.sub..5 B.sub.6              Al        22.8063                                                                               20.8010                                                                               24.1499                                                                               21.7529                                                                               23.3120                                  B         0.4561 0.4160 0.4830 0.4351 0.4662                                   La.sub.2 O.sub.3                                                                         1.0310 0.2088 0.6037 0.4568 --                                       Eu.sub.2 O.sub.3                                                                         --     --     --     --     0.6294                                   BaCO.sub.3                                                                               --     --     --     --     0.7227                                   CsNO.sub.3                                                                               --     --     --     --     --                                       Y.sub.2 O.sub.3                                                                          0.0798 0.5824 0.4105 0.4351 --                                       Crystal Produced                                                                         La.sub..97 Y.sub..03 B.sub.6                                                          --     La.sub..02 Y.sub..98 B.sub.6                                                          --     Eu.sub..86 Ba.sub..14 B.sub.6            __________________________________________________________________________                          Example   15     16                                       __________________________________________________________________________                          Theoretical Crystal                                                                      Eu.sub..8 Y.sub..2 B.sub.6                                                            Eu.sub..9 Y.sub..1 B.sub.6                                    Al        24.6595                                                                               25.8229                                                       B         0.4933 0.5165                                                        La.sub.2 O.sub.3                                                                         --     --                                                            Eu.sub.2 O.sub.3                                                                         1.0604 1.2602                                                        BaCO.sub.3                                                                               --     --                                                            CsNO.sub.3                                                                               --     --                                                            Y.sub.2 O.sub.3                                                                          0.1726 0.0904                                                        Crystal Produced                                                                         Eu.sub..65 Y.sub..35 B.sub.6                                                          Eu.sub..92 Y.sub..08 B.sub.6             __________________________________________________________________________

EXAMPLES 17-18

The following quaternary single-crystal hexaborides were prepared according to the general procedure outlined in Example 1, with the following changes. Example 17 was reacted at 1400° C. and thereafter cooled at a rate of about 8° C./hr. Example 18 was reacted at about 1300° C. and cooled at a rate of about 5° C./hr. The chemical analysis of Examples 17 and 18 indicated the presence of La, Eu, Y, B₆ and La, Eu, Ba, B₆ respectively. However, X-ray and electron probe analysis was unable to give a specific crystal structure.

    ______________________________________                                         QUATERNARY HEXABORIDES                                                         Example      17            18                                                  ______________________________________                                         Theoretical  La.sub..7 E.sub..2 Y.sub..1 B.sub.6                                                          La.sub..5 Eu.sub..4 Ba.sub..1 B.sub.6               Crystal Com-                                                                   position                                                                       Al           19.2982       21.2719                                             B            0.3860        0.4250                                              La.sub.2 O.sub.3                                                                            0.6793        0.5318                                              Eu.sub.2 O.sub.3                                                                            0.2104        0.4680                                              BaCO.sub.3   --            0.1276                                              CsNO.sub.3   --            --                                                  Y.sub.2 O.sub.3                                                                             0.0675        --                                                  ______________________________________                                     

I claim:
 1. A method of fabricating a single crystal metal hexaboride comprising:mixing a metal oxide, nitrate or carbonate compound of the metal component(s) of the hexaboride and boron with sufficient aluminum to dissolve the boron at a reaction temperature of from about 1200° C. to about 1600° C.; heating the mixture to a temperature of from about 1200° C. to about 1600° C. for a sufficient time to form said single crystal metal hexaboride; cooling the mixture; and dissolving said aluminum from said single crystal hexaboride with an acid.
 2. The method according to claim 1 wherein the mixture is cooled at a linear rate of from about 5° to about 20° C. per hour.
 3. The method according to claim 1 wherein the mixture is cooled exponentially according to the formula

    T.sub.f =T.sub.o e.sup.-ατ

wherein T_(f) is the final temperature, T_(o) is the initial temperature, α is the rate of cooling and τ is the cooling time.
 4. The method according to claim 3 wherein the initial rate of cooling is 15° C./hour.
 5. The method according to claim 1 wherein the boron powder is about 1 percent of the weight of aluminum and the reaction mixture is heated to about 1200° C.
 6. The method according to claim 1 wherein the boron powder is about 2 percent of the weight of aluminum and the reaction mixture is heated to about 1350° C.
 7. The method according to claim 1 wherein the boron powder is about 3 percent of the weight of aluminum and the reaction mixture is heated to about 1500° C.
 8. The method according to claim 1 wherein the metal of the oxide, nitrate, or carbonate compound is selected from the group consisting of Cs, Ba, La and the lanthanides.
 9. The method according to claim 8 wherein the lanthanide is selected from the group consisting of La, Ce, and Eu.
 10. The method according to claim 1 wherein the single crystal metal hexaboride has the formula

    L.sub.x M.sub.z R.sub.1-x-z B.sub.6

wherein L is selected from the group consisting of Cs, Ba, La and the lanthanides, M and R are selected from the group consisting of Cs, Ba, Y, La and the lanthanides, x<0 but x≦1, z≧0 but z<1 and 1-x-z<1.
 11. The method according to claim 10 wherein x is equal to 1 and z is equal to
 0. 12. The method according to claim 10 wherein the lanthanides are selected from the group consisting of Ce and Eu.
 13. The method according to claim 10 wherein 1-x-z is equal to
 0. 14. The method according to claim 13 wherein the single crystal metal hexaboride is selected from La_(x) Y_(z) B₆, La_(x) Eu_(z) B₆, La_(x) Ba_(z) B₆, La_(x) Cs_(z) B₆, Eu_(x) Y_(z) B₆ and Eu_(x) Ba_(z) B₆.
 15. The method according to claim 10 wherein x and z are greater than zero.
 16. The method according to claim 15 wherein the single crystal metal hexaboride is selected from the group consisting of La_(x) Eu_(z) Y_(1-x-z) B₆ and La_(x) Eu_(z) Ba_(1-x-z) B₆.
 17. A single crystal metal hexaboride having the formula:

    L.sub.x M.sub.z R.sub.1-x-z B.sub.6

wherein L is selected from the group consisting of La, Cs, Ba, La, and the lanthanides, M and R are selected from the group consisting of Cs, Ba, Y, La and the lanthanides, x>0 but x≦1, z>0 but z<1 and 1-x-z<1 fabricated according to the process of mixing the metal oxide, nitrate or carbonate compound of L, M and R with aluminum and an amount of boron which is soluble in said aluminum at a temperature of from about 1200° to about 1600° C., heating the mixture for a time sufficient to form said single crystal metal hexaboride, cooling the mixture and dissolving the aluminum from said single crystal metal hexaboride with an acid.
 18. The single crystal metal hexaboride according to claim 17 wherein 1-x-z=0.
 19. The single crystal metal hexaboride according to claim 17 wherein x and z are greater than 0 but less than
 1. 20. A ternary single crystal metal hexaboride of the formula:

    La.sub.x M.sub.z B.sub.6

where M is selected from the group consisting of Ba, Eu, Y and Cs, 0<x<1, 0<z<1 and 1-x-z=0.
 21. A ternary crystal metal hexaboride of the formula:

    Eu.sub.x Ba.sub.z B.sub.6

where 0<x<1, 0<z<1, and 1-x-z=0.
 22. A quaternary single crystal metal hexaboride of the formula:

    La.sub.x Eu.sub.z R.sub.1-x-z B.sub.6

where R is selected from the group consisting of Y and Ba, 0<x<1, 0<z<1 and 1-x-z<1. 